1. Field of the Invention
The present invention relates particularly to a wireless transmitting device, wireless receiving device, wireless transmitting method and wireless receiving method by which preamble are transmitted before data.
2. Description of the Related Art
The institute of Electrical and Electronics Engineers (IEEE) is establishing a wireless LAN standard called IEEE 802.11n which aims at a throughput of 100 Mbps or more. In IEEE 802.11n, a technique called multi-input multi-output (MIMO) which uses a plurality of antennas at transmitters and receivers, may be adopted. IEEE 802.11n is required to coexist with the existing IEEE 802.11a. In the MIMO technique, to measure responses (called channel impulse responses) of channel impulse response from a plurality of transmit antennas to each receiving antenna, a preamble as a known sequence must be transmitted from these transmit antennas.
In a preamble proposed by Jan Boer et al. in “Backwards compatibility”, IEEE 802.11-03/714r0 (Jan Boer, “Backwards compatibility”, IEEE 802.11-03/714r0, Section 2, Slide 14 to 19, (URL:ftp://ieee:wireless@ftp.802wirelessworld.com/)), Paragraph 2 “Diagonally loaded preamble”, a short preamble sequence for performing timing synchronization and automatic gain control (AGC) is transmitted from a single transmit antenna. After that, a long preamble sequence for estimating channel impulse response is transmitted from a plurality of transmit antennas. The receiving side performs automatic frequency control (AFC) by using the short preamble sequence and long preamble sequence, and estimates channel impulse response between the antennas. In this manner, the MIMO technique is used in transmission of data signals and the like after that. That is, after the long preamble sequence, a signal field indicating the arrangement of a data signal, e.g., the modulation coding scheme and length of a wireless packet is transmitted, and then the data signal is transmitted.
Jan Boer et al. describe only that the short preamble sequence is transmitted from one antenna and the long preamble sequence is divided into subcarriers and transmitted from a plurality of antennas, and do not describe any signal field transmission method. The preamble proposed by Jan Boer at al. is the same, in a portion from the short preamble sequence to the signal field, as the preamble of IEEE 802.11a standard based on transmission from a single antenna. Therefore, a wireless receiver based on IEEE 802.11a standard which has received the proposed preamble can recognize that the received packet is a wireless packet based on IEEE 802.11a. Accordingly, the proposed preamble allows IEEE 802.11n and IEEE 802.11a standards to coexist on a single wireless station.
The short preamble sequence is transmitted as an orthogonal frequency division multiplexing (OFDM) signal from a single antenna. The long preamble sequence is transmitted, from a plurality of antennas, as different subcarriers divided from one OFDM signal. Likewise, the signal field is transmitted, from the plurality of antennas, as subcarriers divided from one OFDM signal. Since the long preamble sequence is transmitted by dividing one OFDM signal into subcarriers as described above, the receiving side can simultaneously estimate channel impulse response.
In the OFDM receiver apparatus, a received signal is generally demodulated by digital signal processing, so an analog to digital converter is prepared to convert an analog received signal into a digital signal. This analog to digital converter has an allowable level range (called an input dynamic range) permitted to an analog signal to be converted. Therefore, AGC by which the level of a received signal falls within the input dynamic range of the analog to digital converter is essential.
In the preamble by Jan Boer et al., channel estimation is performed by using the long preamble. Since this channel estimation is done by digital signal processing, AGC must be performed by using the short preamble sequence which is a signal before the long preamble sequence. That is, the received level of the short preamble sequence is measured by a receiver connected to each receiving antenna, and the input level of the analog to digital converter is adjusted on the basis of this received level.
Unfortunately, other transmit antennas than the transmit antenna which transmits the short preamble sequence transmit nothing before the long preamble sequence. To receive the long preamble sequence, therefore, AGC must be performed by using the short preamble transmitted from the single transmit antenna. Accordingly, when the receiving side receives the long preamble sequence transmitted from the other transmission antennas or receives a data signal, the received level becomes much higher or lower than the level adjusted by AGC using the short preamble sequence transmitted from the single transmit antenna. If the received level is higher than the upper limit of the input dynamic range of the analog to digital converter, the analog to digital converter saturates. If the received level is lower than the lower limit of the input dynamic range, the analog to digital converter produces a large quantization error. In either case, the analog to digital converter cannot appropriately convert a signal, and this adversely affects processing after the conversion.
Also, since a data signal is transmitted from the plurality of transmit antennas, the changing range of the received level in the interval of the data signal further increases. Accordingly, the problems of the saturation and quantization error of the analog to digital converter described above become significant, and the receiving performance greatly deteriorates.
Generally, a wireless apparatus desirably holds the output level of a transmission signal constant. Assume that the number of transmit antennas is N, and the transmission output is α [watts]. In a wireless communication system obtained by combining the MIMO technique and OFDM, i.e., in a so-called MIMO-OFDM system, the transmission output of a single antenna must be α [watts] for the short preamble sequence because the signal is transmitted from a single antenna. In contrast, for the long preamble sequence, signal field, and data signal, the transmission output of each antenna is α/N [watts] because these signals are transmitted from all antennas.
Accordingly, in a path which transmits the short preamble sequence by using a single antenna, N-fold transmission output is required only to transmit the short preamble. That is, a transmission path for the short preamble produces the redundancy that the specifications of an up converter and power amplifier are required to be able to control the transmission output α [watts] only when the short preamble sequence is transmitted. On the transmission side as described above, a plurality of transmitters corresponding to a plurality of transmit antennas cannot be given equal structures, and this complicates the whole transmitter apparatus. In addition, since the power consumption of the transmitter apparatus strongly depends on the transmission output level, this is not advantageous in achieving low power consumption.